It is thought that neoplasms are a consequence of a series of mutations in host genes that results in loss of the normal cell proliferative control functions. Certain strains of mice, for example AKR, spontaneously develop thymic T-cell lymphomas with very high frequency within about 1 yr of age. The mechanism appears to involve mutagenesis induced by activation or deregulation of one of a number of host genes by near-by insertion of viral sequences, usually of the recombinant mink cell focus-inducing (MCF) class of mouse leukemia virus (MuLV), into host DNA. B cell lymphomas also occur in mice but generally at lower incidence and in much older animals and the mechanism of induction in most cases is not clearly established, either virologically or molecularly. This study examined the lymphoma experience in several families of B-lymphoma prone NFS.V+ mice, derived in this laboratory by breeding chromosomal loci representing ecotropic (mouse cell-tropic) MuLV of high virus, high thymic lymphoma strains onto a virus negative, low lymphoma background, with the aim of fully characterizing and classifying the lymphomas and determining the role of virus in their origin. All hematopoietic lymphomas developing over a 4.5 year period were examined by histopathology for morphologiy and cytology, and many cases were further studied by immunophenotyping using flow cytometry of viable cells or immunohistochemistry on frozen sections, and by analysis of DNA for cell lineage determination, identification of clonal populations, detection of new integrations of ecotropic MuLV and of disruption of a few of the genes previously implicated in lymphomagenesis. In the course of these studies we developed a new classification system for mouse lymphomas, based on the updated Kiel classification of human lymphomas, characterized for the first time in detail the occurrence and stages of mantle zone lymphoma in mice and established the high frequency of lymphoma development in these mice (at least 70% overall incidence within 8 to 20 mo, 90% being B cell lineage). All lymphomas tested contained clonal populations, the majority being monoclonal, and 87% had detectable new non-germline integrations of ecotropic MuLV. In addition, a splenic biopsy and final necropsy protocol was devised to permit the staging of mantle zone and other lymphomas. Finally, no frequent targeting of known oncogenes was detected in screening tests of lymphoma DNAs against 5 probes but, as outlined in other reports, several clones of DNA regions flanking new MuLV integrations have been obtained and are being used for sequencing to identify any known cellular genes and for preparation of probes for screening lymphomas of various types.